 //hdu 2993 斜率优化

typedef long long ll;
 

#define maxn  100030
int n,m;
int a[maxn];
double s[maxn];
int qu[maxn*10];
int main()
{
 #ifndef ONLINE_JUDGE
    freopen("in.txt","r",stdin);
#endif
    int i,j,k,l;
    while(scanf("%d%d", &n,&m)!=EOF)
    {
        s[0]=0;
        for(i=1; i<=n; i++)
        {
            scanf("%d",a+i);
            s[i]=s[i-1]+a[i];
             
        } 
        double ans= 0;
        int head=0,tail=-1;
        double dy1,dy2,dx1,dx2;
        for(i=m; i<=n; i++)
        {
            int now = i-m;
            while(tail-head>=1)
            {
                
                dy1=s[qu[tail]]-s[qu[tail-1]];
                dy2=s[now]-s[qu[tail]];
                dx1=qu[tail]-qu[tail-1];
                dx2=now-qu[tail];
                if(dy1*dx2>dx1*dy2)
                {
                    tail--;
                }
                else     break;
            }
            qu[++tail]=now;
            while(tail-head>=1)
            {
                 
                dy1=s[i]-s[qu[head]];
                dy2=s[i]-s[qu[head+1]];
                dx1=i-qu[head];
                dx2=i-qu[head+1];
                if(dy1*dx2<dx1*dy2)
                {
                    head++;
                }
                else     break;
            }
             
            dy2=s[i]-s[qu[head]];
            dx2=i-qu[head];
         
            ans=max(ans,(double)dy2/dx2);
        }
        printf("%.2lf\n",ans);
    }
}
